Since the introduction of the transistor in 1950, the solid state semiconductor industry has expanded immensely. The foundation of semiconductor technology is based upon silicon which constitutes the primary semiconductor of the industry.
In a similar fashion, solar cell production has evolved from outer space applications to a prospective alternate energy source to replace or supplement dwindling fossil fuel reserves. Once again, silicon is the predominant semiconductor material upon which the technology is based.
Fortunately, silicon, in the form of sand (or quartz) is one of the most abundant elements on earth and is readily available. Reduction of sand with carbon in a conventional arc furnace is a simple and inexpensive means of producing metallurgical grade silicon. Considering both material and processing costs, metallurgical grade silicon can be produced for as little as sixty cents per kilogram.
Unfortunately, even the purest sand and carbon sources produce metallurgical silicon having an impurity content in excess of an order of magnitude too large for semiconductor applications. Therefore, in the conventional processing procedure to produce silicon of a suitable purity for solar cell or transistor applications, expensive and complicated purification processes are employed to lower impurity levels. These processes typically involve the following steps: pulverizing the metallurgical grade silicon to finely divide the material; chlorination at high temperatures to produce trichlorosilane (and attendant impurity compounds such as metal chlorides); purifying the trichlorosilane by distillation and similar techniques to separate the trichlorosilane from the aforementioned contaminants; further purification (although accomplishable concurrent to the above step of distillation) of the trichlorosilane to remove metal compounds and other contaminants; finally, a chemical vapor deposition to reduce the purified trichlorosilane with hydrogen to produce polysilicon rods which are suitable as feed stock for single crystal growth. The process of purification increases the expense of production by almost two orders of magnitude.
These purification processing costs, though increasing the overall cost of most semiconductor devices, have not effected an intrinsic impediment to the growth and development of that industry. This is principally attributable to the fact that the total area which each transistor or integrated circuit device occupies on a silicon wafer is very small, therein reducing the relative contribution of the silicon itself to the total device fabrication cost.
The solar cell industry, unlike its semiconductor counterpart, does face an inherent price barrier in fabrication costs. At the present cost of about fifteen dollars a peak watt, solar energy has very little viability as an alternative to conventional fossil fuel or even to more exotic nuclear fission produced electricity. As such, solar energy, a clean and pollutant-free source of energy, has been economically limited to remote areas and similarly specialized applications. Technical advances are being made to improve solar cell efficiencies and fabrication processes to result in a net reduction in solar cell costs. However, to achieve a viable cost basis for producing electricity by way of silicon solar cells, a substantial reduction in the material costs is necessary.
To this objective, the present invention provides a method for producing solar grade silicon at a substantial cost reduction over the conventional techniques.
The outer coating of commonly grown rice is comprised primarily of cellulose, lignin and silica. These rice hulls, being indigestible as grown, are a bothersome byproduct of the rice production industry which typically incinerates them at significant expense or simply dumps them into vacant fields. The Quaker Oats Company has developed a commercial application of extracting furfural from the hulls, and Silag, a subsidiary of Exxon Corp, has explored the manufacture of silicon carbide whiskers from rice hulls based on the technology disclosed in U.S. Pat. No. 3,754,076. But generally speaking, these rice by-products have been considered waste materials. The disposal of these hulls has become of significant concern to the rice industry. Many rice producing states have banned or severely restricted rice hull burning because of the toxic by-products produced. Dumping the spent hulls is similarly objectionable due to transporting costs and attendant environmental disfigurement. California alone produces more than two hundred thousand tons of rice hulls a year, and incinerating is not longer permitted there.
One need examine no further the dilemma facing the rice industry to appreciate that rice hulls are a readily abundant, low or no-cost starting material if a beneficial use can be found.
It is therefore the object of the present invention to utilize these rice hulls as a starting material in a unique purification and reduction process to inexpensively produce solar grade silicon, useful in the fabrication of silicon solar cells and similar semiconductor devices.
The purification process, entailing a sequence of leaching and pyrolyzing the rice hulls, provides a high purity feedstock for the subsequent processing into solar grade silicon. This purification produces a feedstock having a significantly reduced overall impurity level and most importantly having a more than one hundred fold reduction in the level of impurities which most detrimentally affect the semiconductor properties of the intended end product.
The reduction process, entailing a sequence of adjusting carbon content and reducing the siliceous material, utilizes the purified feedstock to provide elemental silicon of sufficiently high purity to be used in conventional solar cell processing.